Effect Of Bubbles On Granular Structure And Characteristics Of Anammox Granular Sludge

As a new biological nitrogen removal process,Anammox has the advantages of no need for organic carbon,no need for oxygen,low sludge yield,and low cost.Its maximum nitrogen removal load can reach 9.5kg/（m³·d）.It is a high efficiency and low consumption of nitrogen removal process and has great application prospect.Anammox granular sludge is favored by environmental engineering circles for its advantages of large biomass,good settlement and strong impact resistance.After supersaturated,dinitrogen generated by Anammox granules accumulates in the granule or on the surface in the form of gas.Once the total amount of gas exceeds the total amount of gas stored in the granules,the gas will escape from the granule in the form of bubbles.The traditional view was that the bubble formation of anammox granule played an important role in the stratification of anammox reactor sludge bed and granule floating,which had a great impact on the stable operation of the reactor.At present,the research on the gas generation process of granular sludge was mainly focused on the macroscopic gas productivity and reactor performance,while the growth and migration of CH₄ or N₂ bubbles were only concerned in aquatic systems.In anammox and other gas-producing wastewater biological treatment systems,the complete process of bubble generation and escape,as well as the mechanism of bubble interaction with granular sludge,had not been systematically and thoroughly studied.In this study,by establishing a systematic characterization method for the bubbles generated by granular sludge,the influence mechanism of bubbles on the structure and characteristics of granular sludge was analyzed,which provided theoretical basis for further strengthening the structure and function of granular sludge and guiding the efficient and stable operation of Anammox and other gas-producing granular sludge reaction systems.The main research contents and results of this study are as follows:（1）A systematic characterization method for the dynamic process of bubble generation in granular sludge was established.In the reaction process of anammox granular sludge,the method of microfluidic combined with fluorescence microscopic imaging was used to observe the nanobubbles（NBs）,which proved the existence of NBs in the field of microorganism,and the characteristics of NBs such as particle size distribution,concentration and surface stability were preliminarily explored.Furthermore,the presence of NBs in the real reactor was confirmed.Using high-speed camera and image processing technology,it was observed that most of the microbubbles generated by anammox granular sludge were 25-300μm,with an average particle size of 141.87-189.4μm.And the bubble density inside the reactor could reach up to 8840-10600 bubbles/m³.The single bubble on the surface of the granule was cyclically generated and grown,and the bubbles could gradually increase from no to 158.86μm within 60 s and then peeled off the surface of the granule.The gas produced by granular sludge was mainly N₂（above 99%）produced by anammox or denitrification with a small amount of CO₂（0.43%）and N₂O（0.01%）.（2）The action mechanism of granular sludge bubbles on the formation of microporous structure inside granular sludge and enhanced mass transfer of substrate and products was revealed.The gas production and structure of anammox granules with different particle sizes were characterized.The results showed that the abundance of anaerobic ammonium oxidation bacteria（AAOB）,the ratio of live bacteria,and the anammox activity of largely and jointly determined the production of NBs and microbubbles.Large granules with high activity had a relatively compact spatial structure and low porosity,which largely affected the permeability and molecular diffusion processes inside the granules.The stable presence of NBs could scour the nearby over-secreted EPS to prevent the clogging of nanopores,and also avoid the growth and reproduction of microorganisms inside the granules and the excessive extrusion of hydraulic shear,thus ensuring the abundant nanopore structure and high specific surface area around the cells.（specific surface area,SSA）,which further promoted the full contact between bacteria and substrates and the mass transfer efficiency of cell aggregates,fully improved the utilization of microorganisms inside the granules,and played an important role in the efficient and stable reaction of the granules.The pore structure of the granules was formed under the combined action of the granule’s own microbial growth,water flow scouring,and the generation and migration of bubbles.The balance of the three effects shaped the granule’s high activity and high mass transfer characteristics.（3）The influence mechanism of bubble on the morphology of granular sludge was summarized.The gas production,structure,and morphology of anammox granules under different loads were characterized.The results showed that due to the increase of nitrogen loads,the microbial community changed,and the dominant bacteria of AAOB changed from Candidatus-Jettenia to Candidatus＿Kuenenia.Candidatus＿Kuenenia had better tolerance to high concentration of matrix and salinity,and has stronger EPS secretion ability.And The conversion between heterotrophic bacteria（Clostridium＿sensu＿stricto＿13,OLB13,SJA-28＿norank,etc.）that mainly used EPS as substrates significantly increased the EPS content in the granules under high loads,especially the increase of polysaccharide components,which in turn maked the gas channel of the granular sludge blocked during the gas production process.In addition,the increase in the production of microbubble under high loads further aggravated the deformation of the gas channel of the granular sludge,which further changed the internal pore structure of the granular sludge.the volume of the cavity increased,and the surface morphology also changed.The convexity also further increased,and the sphericity decreased,which was not conducive to the stability of the granules.（4）The bubble-driven updating mechanism of surface or interior pore of granular sludge was derived.The surface characteristics,microbial activity,and microbial community structure of aggregates in the supernatant and their own granules with different sizes were compared and analyzed.The results showed that the formation,growth and migration of microbubbles and NBs played a leading role in the renewal process on surface and interior pores the granules.Bacteria such as Shinella,Comamonas,Paracoccus,Rhizobiaceae＿unclassified,Hydrogenophaga,Pseudomonas,Thermomonas,Thauera,and Flavobacterium were more susceptible to bubble-driven exfoliation into the supernatant.Surface properties such as contact angle（CA）,zeta potential（ZP）,adhesion force,and live bacteria ratio of aggregates in the supernatant were reduced compared to the granules.When the shear force generated by bubble generation and migration is greater than the adhesion force between loose aggregates and granules,the microbial aggregates with less active and surface EPS and poor adhesion properties were exfoliated from surface or pores of the granules into the supernatant,which was more conducive to granule stability and efficient nitrogen removal.（5）The influence mechanism of bubbles on the stability of granular sludge and its settleability was summarized.The relationship between bubble characteristics,microbial properties,granule surface properties and pore structure of anammox granules,as well as the causal relationship between them and granule stability and settling property were analyzed.The results showed that NBs indirectly determined the stability of the granules by directly affecting the EPS content inside the granules and the structure of nanopores.The stable NBs and the nanopore structure around the cells were directly determined by the good cross-linking structure inside the anammox granules.The ultra-high density of gas molecules confined in the NBs may also increased the strength and stability of the granules.In addition,Exfoliation of the EPS induced by the NBs from the inside of the granules might also be beneficial for the aggregation and stability of anammox granules.Differences in nitrogen loading led to differences in microbial community structure,EPS composition,and gas production activity.The differences in the micro-bubble characteristics and EPS composition led to changes in the internal pore structure of the granules,which in turn affected the stability of the granules.Under high nitrogen loading,the high gas production activity and high EPS concentration caused the gas channel blockage,a large number of cavities were formed inside the granules,and the biomass cross-linked structure that maintained the granule stability was destroyed,making the granules deform.The shear resistance and viscoelasticity of granules were seriously reduced,which affected the stability of granules.Under different nitrogen loadings,the characteristics of micro-bubbles and the differences in the microbial surface properties of granular sludge（such as EPS）regulated the density of granular sludge and the apparent morphology of the granular sludge by affecting the pore structure inside the granular sludge and then played a leading role in the settling performance.Compared with the settled granules,although the floating granules also had obvious biomass cross-linking structure and inorganic crystal nuclei,the cross-linking structure was weak,and large cavities appeared inside the granules.When the density of granules was less than that of water,the floatation appeared.（6）The influence mechanism of bubbles on the sludge bed structure was elucidated.The relationship between bubble characteristics of anammox granules,granule bed structure,and mass transfer characteristics was analyzed.The results showed that with the increase of nitrogen load,the gas discharge rate inside the granules was affected,and the bubbles were more likely to stay inside the granules,resulting in an increase in the proportion of the internal cavity of the granules and the apparent density of the granules.The surface of the medium-loaded granules had the best hydrophobicity,which resulted in the maximum adhesion force of 11.34 m N between bubbles and granules.The bubbles were more likely to stay in the gap between the granules in the sludge bed.The largest,the loosest sludge bed was more conducive to the contact and reaction between granules and substrates.The generation and stagnation of bubbles inside the sludge bed greatly increased the permeability coefficient.The permeability coefficient of the sludge bed of the R4 reactor after gas generation increased the most,which was 1.97 times of that before gas generation.Moreover,the contribution of gas generation process to the reduction of energy consumption of R4 reactor sludge bed reached 74.17%,which greatly reduced the pressure drop and energy consumption of its own sludge bed.The change of microbial community structure inside the granules resulted in the difference of gas-producing activity and the change of granule characteristics（such as microbial surface characteristics and granule morphology）,which in turn affected the characteristics and adhesion characteristics of micro-bubbles.The difference in characteristics of micro-bubbles and their adhesion characteristics affected the permeability of the sludge bed and the difference in energy consumption inside the reactor by affecting the density of the granules and the porosity and expansion ratio of the sludge bed,which ultimately affected the performance and stability of the reactor.